Circuit pattern formation using metal nanoparticles having a particle size of nanometer order is known, and a number of techniques have recently been proposed in which a circuit is drawn or printed directly on a variety of substrates using a conductive metal ink (a metal ink containing metal nanoparticles) by means of an inkjet printer or a dispenser, and the applied ink is fired to form conductor wiring or electrodes. Circuit pattern formation on a variety of substrates by firing metal nanoparticles at low temperature is exemplified by the proposal of Patent Document 1. Circuit formation by inkjet printing a conductive metal ink is disclosed, e.g., in Patent Document 2.
The technique in which a circuit pattern is directly printed on a substrate by inkjet printing is attracting attention as a promising process allowing for great reduction of production cost because of a fewer number of steps involved and less waste from the steps than the generally spread, conventional circuit pattern forming technique utilizing photolithography. The conventional technique relying on photolithography is exemplified by the one disclosed in Patent Document 3.
We have thus seen the technological changes from photolithographic techniques into inkjet printing or dispenser techniques, which have enabled more convenient and less expensive circuit formation on a substrate. Nevertheless, the circuit formation technique using a conductive ink by inkjet printing or dispensing using a dispenser has not been widely spread primarily for the following reasons.    (i) The resulting conductor film lacks adhesion to various substrates, failing to fulfill fundamental characteristics required of a circuit board.    (ii) The resulting conductor film lacks sufficient surface smoothness. A circuit usually has a laminate structure including a base layer, so that it would have limited applicability in various respects unless the conductor film has sufficient surface smoothness. For example, a different component layer provided on a rough surface of a conductor film would be influenced by the underlying surface roughness and fail to maintain good thickness uniformity.
The problem (i) is believed to be largely caused by the characteristics of a dispersion medium used in a conductive ink. It is considered that the adhesion of a conductor film formed by firing applied ink to a substrate is governed by the chemical reaction between a binder component present in the dispersion medium and the substrate. The problem (ii) is believed to be caused by the characteristics of both the metal powder (metal particles) and the dispersion medium constituting a conductive ink. It is obvious that coarse metal particles make it impossible to form a conductor film with smooth surface. In addition to this, if the dispersion medium vigorously vaporizes and escapes from the inside of the conductor film during firing, it easily results in a failure to form a smooth film surface.
With respect to the binder component in ink, Patent Document 4 discloses using a silane coupling agent having a mercapto group or an amino group. The metal particles recited for use in the ink is gold, silver, platinum, copper, or palladium, but does not include nickel. Furthermore, the ink of Patent Document 4 is pasty with a relatively high viscosity and cannot be seen as suited to inkjet printing.                Patent Document 1 JP 2002-334618A        Patent Document 2 JP 2002-324966A        Patent Document 3 JP 9-246688A        Patent Document 4 JP 2004-179125A        